Field of the Invention
The present invention relates to an optical data transmitting apparatus and a method for an optical data transmission system using optical fiber.
Recently, in optical fiber data transmission, intensity modulation direct detection systems have been widely used and wavelength-multiplexing technology has been employed for increased data transmission capacity. In this type of system, a combination of group velocity dispersion (GVD) and self phase modulation (SPM) occurring in an optical fiber causes waveform distortion, resulting in degraded data transmission quality.
GVD is a phenomenon in which propagation speed through optical fiber varies with wavelength. Modulated signals suffer from waveform distortion because of its wide signal spectrum. Recent technology provides a compensation element for waveform distortion due to GVD. Such a compensation element, however, is expensive. Thus, there have been proposals for modulated signals which are less affected by GVD. The narrower the signal spectrum, the less the signal is affected by GVD. Thus, improvement may be expected if the signal spectrum is narrowed. Optical duobinary modulation is one approach. JP-A 8-139681, which is deemed to be the Japanese equivalent of U.S. Pat. No. 5,543,952 (Yonenaga et al.), and K. Fukuchi et al. OFC ""97 Technical Digest, ThH3, 1997 teach optical data transmission utilizing such optical duobinary modulation. Duobinary signals fall within a category of partial response signals. Three-level duobinary signals are known, and have three levels of amplitude. According to the optical duobinary modulation, three levels are allocated to three states xe2x80x9c+1xe2x80x9d, xe2x80x9c0xe2x80x9d, xe2x80x9cxe2x88x921xe2x80x9d using phase and amplitude of light. The state xe2x80x9c0xe2x80x9d indicates that the intensity level is zero. The state xe2x80x9c+1xe2x80x9d and xe2x80x9cxe2x88x921xe2x80x9d indicate that the intensity level is one, but the phases are inverted from each other. At the receiver, this optical duobinary modulated signal is decoded into the original two-level digital signal by normal direct detection. During propagation through optical fiber, the signal band is left in a compressed state and waveform distortion due to GVD is low.
SPM is another cause of degradation of data transmission quality. SPM is a phenomenon in which phase modulation is applied to a signal as its light intensity varies with time. This problem is unavoidable if intensity modulation is employed. The above-mentioned optical duobinary signal is the same as the ordinary intensity modulated signal in that it uses light intensity. Thus, suppression of SPM cannot be expected. SPM increases signal spectrum bandwidth although it does not alter the signal waveform, thereby limiting the density of wavelength multiplexing. SPM cooperates with GVD to induce a combined SPM+GVD effect.
As one approach to reduce SPM, polarization modulation has been proposed. In polarization modulation, light signals are allocated to different polarization states of light. As the overall intensity is unaltered, the effect of SPM is less, thus suppressing degradation of data transmission quality. JP-A 1-208920 (Fukaya) discloses an optical fiber data transmission employing such polarization modulation.
The SPM+GVD effect in question is a phenomenon in which during propagation through an optical fiber, SPM occurs concurrently with the occurrence of waveform distortion, and thus the dispersion compensation technique cannot restore the waveform. If SPM occurs concurrently with waveform distortion, the light signal is subjected to frequency chirping. The presence of GVD causes the chirping to induce further waveform distortion. This further waveform distortion induces new SPM. Thus, the waveform distortion is different from distortion in the absence of SPM, and this is the reason why the dispersion compensation technique is ineffective to restore the waveform distortion under the presence of SPM. The effect of GVD+SPM is thus considered to be the major obstacle to the extension of data transmission distances in optical fiber data transmission to any distance exceeding 1000 kilometers.
From preceding description, it is to be understood that the conventional duobinary signal is ineffective against SPM, and polarization modulation is ineffective to prevent GVD, although the former is effective against GVD and the latter is effective against SPM.
The present invention aims to provide a signal combining the merits of the conventional duobinary signal and polarization modulation.
According to one aspect of the present invention, there is provided an optical data transmitting apparatus comprising:
a code encoder transforming a binary input data signal to obtain a duobinary signal having a center level and two extreme levels;
a carrier source of a carrier light;
an optical modulator modulating phase and polarization of said carrier light in response to said duobinary signal to obtain a modulated output light,
said modulated output light having first polarization state that corresponds to said center level of said duobinary signal, and second and third polarization states that correspond to said two extreme levels, respectively,
said second and third polarization states sharing a common plane of polarization that is orthogonal to a plane of polarization of said first polarization state,
said second and third polarization states having phases that are inverted to each other.
According to another aspect of the present invention, there is provided an optical data transmitting apparatus comprising:
a code converter transforming a binary input data signal to obtain a pair of mutually complementary duobinary signals;
a carrier source of a carrier light;
an intensity modulator separating said carrier light into first and second carrier lights, altering phases of said first and second altered carrier lights in response to said pair of mutually complementary duobinary signals, respectively, combining said phase altered first and second carrier lights to obtain a pair of mutually complementary interference output light; and
means for combining said pair of mutually complementary interference output light beams in mutually orthogonal polarized relation.
According to still another aspect of the present invention, there is provided an optical data transmission apparatus comprising:
means for separating a binary input signal into a first altered data signal and a second altered data signal;
a polarization modulator modulating polarization of said carrier light in response to said first altered data signal to obtain polarized carrier light;
a pre-coder transforming code of said second altered data signal to obtain a coded second altered data signal; and
a phase modulator modulating the phase of said polarized carrier light in response to said coded second altered data signal.
According to a further aspect of the present invention, there is provided an optical data transmitting apparatus comprising:
a carrier source for a carrier light;
means for separating a binary input data signal into a first data signal and a second data signal;
a polarization modulator modulating polarization of said carrier light in response to said first data signal to obtain polarization modulated carrier light;
a pre-coder transforming code of said second data signal to obtain a coded second data signal; and
a phase modulator modulating the phase of said polarization modulated carrier light in response to said coded second data signal.
According to a still further aspect of the present invention, there is provided an optical transmitting apparatus comprising:
a carrier source of carrier light;
means for separating said carrier light into first and second orthogonally polarized lights, phase modulating said first polarized light in response to a first driving signal to obtain a phase modulated first polarized light, phase modulating said second polarized light in response to a second driving signal to obtain a phase modulated second polarized light, and combining said phase modulated first and second polarized lights; and
a driving signal generator inputting a binary input data signal to generate said first and second driving signals.
According to a further aspect of the present invention, there is provided an optical transmitting apparatus comprising:
a carrier source of carrier light;
a polarization modulator modulating polarization of said carrier light,
said polarization modulator being provided with means for inputting a duobinary signal as a modulator driving signal,
said polarization modulator being an optical azimuth rotator and operative to optically rotate the polarization of said carrier light in response to said modulator driving duobinary signal such that said carrier light has an opposite phase in response to said modulator driving signal being at the maximum phase or the minimum phase.
According to yet another aspect of the present invention, there is provided an optical data transmitting method comprising the steps of:
transforming a binary input data signal to obtain a duobinary signal having a center level and two extreme levels; and
modulating the phase and the polarization of a carrier light in response to said duobinary signal to obtain a modulated output light,
said modulated output light having a first polarization state that corresponds to said center level of said duobinary signal, and second and third polarization states that correspond to said two extreme levels, respectively,
said second and third polarization states sharing a common plane of polarization that is orthogonal to a plane of polarization of said first polarization state,
said second and third polarization states having phases that are inverted from each other.
According to a further aspect of the present invention, there is provided an optical data transmitting method comprising the steps of:
transforming a binary data signal to obtain a pair of mutually complementary duobinary signals;
separating a carrier light into a first carrier light and a second carrier light;
modulating phases of said first and second carrier lights in response to said pair of mutually complementary duobinary signals, respectively, to obtain phase modulated first and second carrier lights;
combining said phase modulated first and second carrier lights to interfere with each other to obtain a pair of mutually complementary interference output lights; and
combining said pair of mutually complementary interference output lights in mutually orthogonal polarized relation.
According to a further aspect of the present invention, there is provided an optical data transmitting method comprising the steps of:
separating a binary input data signal into a first data signal and a second data signal;
inputting said first data signal to obtain a pair of mutually complementary interference output lights;
transforming a code of said second data signal to obtain a coded second data signal""
inputting one of said pair of mutually complementary interference output lights and said coded second data signal to modulate the phase of said one interference output light in response to said coded second data signal to obtain a phase modulated interference output light; and
combining said phase modulated interference output light and the other of said pair of mutually complementary interference output lights in mutually orthogonal polarized relation.
According to a further aspect of the present invention, there is provided an optical data transmitting method comprising the steps of:
separating a binary input data signal into a first data signal and a second data signal;
modulating the polarization of a carrier light in response to said first data signal to obtain a polarization modulated carrier light;
transforming a code of said second data signal to obtain a code transformed second data signal; and
modulating the phase of the polarization modulated carrier light in response to said code transformed second signal.
According to a further aspect of the present invention, there is provided an optical data transmitting method comprising the steps of:
separating a carrier light into first and second orthogonally polarized lights;
phase modulating said first polarized light in response to a first driving signal to obtain a phase modulated first polarized light;
phase modulating said second polarized light in response to a second driving signal to obtain a phase modulated second polarized light;
combining said phase modulated first and second polarized lights; and
inputting a binary input data signal to generate said first and second driving signals.
According to a further aspect of the present invention, there is provided an optical data transmitting method comprising the steps of:
transforming a binary input data signal to obtain a duobinary signal as a modulator driving signal; and
rotating a polarization of a carrier light, using an optical azimuth rotator, in response to said modulator driving signal such that said carrier light has an opposite phase in response to said modulator driving signal being at the maximum or the minimum.